Get to Victoria Station. It's crowded and wildly tourist-infested, but it's also a central hub to all points north, south, east and west. Victoria Station is easily accessible from Heathrow airport via the tube's Victoria line, for just 4 GBP. From Victoria station, you can enjoy a delicious beverage and pastie while researching your options. The list of possibilities is endless: Oxford, Cambridge, Brighton, Gloucester and hundreds of other destinations.
Ask the attendants at the station where to stand on line for a trip to Stonehenge, Bath or Windsor Castle. There are myriad other popular day trips outside of London, some of which combine a few destinations in a day. You can't go wrong with Stonehenge, though, so why not start there.
Confer with your hotel receptionist about where to go. Hotels in London are usually spilling over with information on day trips outside the city. For example, Gray Line bus service will offer tours of Stonehenge, Windsor and Bath and pick you up at your hotel if it's on their list of serviced properties. If not, a pickup point is very likely near to where you are, even a neighboring hotel.
Travel by rail almost anywhere in Britain, including Wales and Scotland. Check the National Rail service timetables, routes and prices at the link below. Not only is train travel ubiquitous and fairly priced in England, but the train ride itself is part of the adventure. As you pass by ancient castles set on verdant pastures or cobble-stoned, thatched-roof villages, you'll be glad you bought a train ticket.If you are taking the train with hopes of going to Stonehenge or Bath, check information for connecting service. For example, to get to Stonehenge by rail, start at Waterloo station in London and get off at Amesbury Station. From there it's about 9 1/2 miles to your destination, so you can take a taxi or connecting bus service, depending on your budget.
Step outside your comfort zone and say bollocks to the budget! While most Londoners wouldn't think of going to Paris just for the day, you might not have the option to enjoy a longer visit. If this is the case, take the train from St. Pancreas station in London to Paris, Gare du Nord, also the main Metro hub in Paris--hich is very convenient. You will want to book your ticket on the Eurostar, which will cost you about $450 or more round-trip. Yes, this is pricey, but so was your vacation!
Fly away for the day. To most Americans, who are used to thinking big, the idea of flying to another country for a day sounds not only tiring but incredibly expensive, if it's even possible. But think like a Brit. They're flying all over the place on one of a number of low-cost carriers. The sky is, quite literally, the limit here. A good choice would be Dublin, Shannon, the aforementioned Paris, Brussels or Edinburgh, for example, rather than something more than an hour or so away. The issue is more with the delays at airports rather than the actual flight time.